This invention relates to an image forming apparatus such as a digital copying machine or laser printer using a beam light scanning device, for example, for simultaneously scanning and exposing a plurality of beam lights on a single photosensitive drum to form a single electrostatic latent image on the photosensitive drum.
Recently, for example, various types of digital copying machines for forming an image by effecting the electrophotographic process and the scanning and exposing process by use of beam lights have been developed.
Further, in recent years, in order to further enhance the image forming speed, a multi-beam type copying machine, that is, a digital copying machine for generating a plurality of beam lights and simultaneously scanning plural lines by use of the plurality of beam lights has been developed.
In the above multi-beam type copying machine, an optical system unit is provided as a beam light scanning device which includes a plurality of semiconductor laser oscillators (which are hereinafter referred to as laser oscillators) for generating beam lights, a polygonal rotating mirror such as a polygon mirror for reflecting the beam lights output from the plurality of laser oscillators to a photosensitive drum by use of galvanomirrors and scanning the reflected beam lights on the photosensitive drum, and collimator lens and f-.theta. lens as main components.
As an image forming apparatus having the above beam light scanning device, apparatuses are proposed in U.S. patent application Ser. No. 09/150,705 (Jpn. Pat. Appln. KOKAI Publication No. 11-95142, Japanese Patent Application No. 9-257351), U.S. patent application Ser. No. 09/439,088 (Japanese Patent Application No. 10-323872), U.S. patent application Ser. No. 09/470,884 (Japanese Patent Application No. 10-365611). In this proposal, the position control operation of each beam light and the light amount (power) control operation are effected based on a detection output from a beam light position detecting sensor. By the above control operations, the misalignment of images due to the positional deviation of the beam lights of an image and irregularity of the image due to a difference in the light amounts (powers) of the beam lights can be prevented.
As shown in FIG. 1, the galvanomirror is constructed by a magnet fixing base, magnet, bobbin, coil, torsion bar, and mirror.
In the galvanomirror, the mirror is moved in the sub-scanning direction by controlling the direction and amount of a current flowing in the coil to control the position of the beam light in the sub-scanning direction. This utilizes force caused between the coil and the magnet based on the Fleming's left-hand rule.
In the multi-beam control operation, a plurality of beam lights are not simultaneously controlled but the beam lights are sequentially controlled for each beam light. In this case, in order to omit a variation in the surface precision of a polygon motor for rotating the polygon mirror, the sub-scanning control operation is effected by use of an average value of the eight surfaces of the polygon mirror (corresponding to one rotation of the polygon mirror).
With the above apparatus, in order to compensate for a drift of the galvanomirror, the multi-beam control operation which is disclosed in U.S. patent application Ser. No. 09/470,884 (Japanese Patent Application No. 10-365611) is effected at the turn-ON time of the power supply, at the end of the printing process, in the READY state, and in a paper printing-paper printing time interval.
As shown in FIG. 2, at the turn-ON time of the power supply, the rough adjustment of the sub-scanning position (ST1), control of the beam power (ST2), fine adjustment of the sub-scanning position (ST3), control of the main scanning position (ST4), and control of the main power (ST5) are sequentially effected from the time immediately after the turn-ON of the power supply.
At the end of the printing process, the pre-printing preparation control is effected.
As shown in FIG. 3, in the pre-printing preparation control, the rough adjustment of the sub-scanning position (ST6), fine adjustment of the sub-scanning position (ST7) and control of the main scanning position (ST8) are sequentially effected.
That is, the rough adjustment of the sub-scanning position is first made and then the beam light is moved with large steps (100 steps.apprxeq.a variation 176 .mu.m in the image surface position) until the beam light comes onto the surface of the beam light position detecting sensor in order to move the beam light onto the beam light position detecting sensor. Next, the fine adjustment of the sub-scanning position is made to control the beam light which has been brought onto the surface of the beam light position detecting sensor by the rough adjustment and set the same within a to-be-controlled target range of .+-.2.7 .mu.m. After this, control of the main scanning position is effected.
As shown in FIG. 4, as the control operation in the READY state, a sub-scanning control process (ST11, ST12) in the READY state for each of four beam lights and a control process (ST13, ST14) in the READY state for each of the four beam lights are sequentially effected. The sub-scanning control process in the READY state and the main scanning control process in the READY state are repeatedly effected until an instruction such as start of the printing process is issued (ST15).
After the printing process is started and when the printing process is terminated, the pre-printing preparation process (ST17) is effected.
That is, as shown in FIG. 5, in the sub-scanning control process in the READY state, a to-be-controlled beam light is first emitted (ST21), a specified value in the preceding cycle is given to the galvanomirror (ST22), whether or not the beam light lies within the target range of (.+-.2.7 .mu.m) is determined (ST23), and if it lies within the target range, no process is effected. If it does not lie in the target range, whether it lies above or below the target is determined (ST24), and it is moved by one step in a direction closer to the target (ST25, ST26). The control process is sequentially effected for each of four beam lights (from ST21 to ST27).
Next, as shown in FIG. 6, in the main scanning control process in the READY state, a beam light is emitted (ST31) with the previous specified value (corresponding to a dot value: one pixel (42.3 .mu.m), tap value: approx. 1/10 dot (approx. 42.3 .mu.m), whether the sensor response is obtained by eight times (corresponding to eight surfaces of the polygon mirror) or not is determined (ST32). If the response is obtained by eight times, the tap value is changed to reduce the time for emission of the beam light by one tap (ST33) and if the response is not obtained by eight times, the tap value is changed to increase the time for emission of the beam light by one tap (ST34). In a case where the response is not obtained by eight times even if the time for emission of the beam light is increased to a maximum value, an error is displayed on the main body (ST35). The control process is sequentially effected for each of four beam lights (from ST31 to ST36).
In the control process in the paper printing-paper printing time interval, as shown in FIG. 7, like the READY control process, a sub-scanning control process (ST41, ST42) for each of the four beam lights and a main scanning control process (ST43, ST44) for each of the four beam lights are effected.
The above control process is different from the READY control process in that the control operation is repeatedly effected in the READY control process until a specification such as start of printing is issued, but in the above control process, the control operation is effected only once.
During the printing process, as shown in FIG. 8, a deviation of each of the beam lights in the main scanning direction is corrected by use of the dot value and tap value to adjust the print start timing.
The galvanomirror moves the mirror in the sub-scanning direction by controlling the direction and amount of a current flowing in the coil so as to control the sub-scanning position of the beam light. This utilizes force caused between the coil and the magnet based on the Fleming's left-hand rule. By energizing the coil, heat is generated in the coil of the galvanomirror. Since the atmospheric temperature of the galvanomirror is changed due to heat generation in the coil by energization of the coil, the viscosity of a damping agent is changed and the magnitude of magnetic force of the magnetic circuit fluctuates, and a drift phenomenon that the mirror is rotated (shifted) in the sub-scanning direction occurs.
As shown in FIG. 9, the drift of the galvanomirror gives an influence not only on the sub-scanning direction but also on the main scanning direction, and it may cause a positional deviation in the main scanning direction by the same distance as in the sub-scanning direction at maximum. As shown in FIG. 1, this is because the galvanomirror moves not only in the vertical direction with respect to the sub-scanning direction but also in the oblique direction by distortion of the torsion bar of the galvanomirror. Further, this is due to the precision at the assembling time which is caused by mounting the mirror in an inclined state, for example. For this reason, it is necessary to take the influence by the drift of the galvanomirror into consideration also in the main scanning direction.
As shown in FIG. 10, there is a possibility that the beam light on the image surface (on the photosensitive drum) will be deviated by 50 .mu.m at maximum from the reading position of the optical sensor in the sub-scanning direction for approx. one second (0.87 second) due to the drift of the galvanomirror by energization.
As described above, since the beam light on the image surface may be deviated in the main scanning direction by the same amount as in the sub-scanning direction by the influence of the drift of the galvanomirror by energization, there occurs a possibility that it will be deviated by 50 .mu.m also in the main scanning direction.
To-be-controlled target ranges of the image surface in the main scanning direction and sub-scanning direction are respectively set within 10 .mu.m in the main scanning direction and within .+-.2.7 .mu.m in the sub-scanning direction. Therefore, the deviation of 50 .mu.m in the main scanning direction and sub-scanning direction causes the beam light to be largely deviated from the to-be-controlled target range, thereby causing a possibility that a problem occurs in the image.
In order to prevent the above problem, the multi-beam control operation is effected at the turn-ON time of the power supply, at the end of the printing process, in the READY state, and in the paper printing-paper printing time interval.
However, during the printing process, since the printing process is effected based on the control result described before, neither the control operation in the sub-scanning direction nor the control operation in the main scanning direction is effected. In the control process in the paper printing-paper printing time interval at the time of printing, if the paper size is A3 or LD, the control process is effected at control intervals of one time for approx. two seconds in the main scanning direction and only the control operation of one tap: (approx. 1110 dots (approx. 4.2 .mu.m) can be effected. Likewise, the control operation is effected at control intervals of one time for approx. two seconds also in the sub-scanning direction and only the control operation of one step=(approx. 2 .mu.m) (FIG. 11) can be effected.
Therefore, if the deviation of 2.0 .mu.m which is the limit of correction in the sub-scanning direction or more or 4.2 .mu.m which is the limit of correction in the main scanning direction or more occurs due to the drift or the like, the position of the beam light cannot be controlled by the control process in the paper printing-paper printing time interval and there may occur a possibility that a problem occurs in the image.
Further, if a deviation larger than .+-.2.7 .mu.m which is the to-be-controlled target range in the sub-scanning direction occurs or if a deviation larger than 4.2 .mu.m which is the to-be-controlled target range in the main scanning direction occurs, a possibility that a problem occurs in the image at the rear end of paper may occur. For this reason, it is required to effect the control process in the sub-scanning direction and the control process in the main scanning direction during the printing process.